1. Field of the Invention
This invention relates to lithographic processes, and, in particular, to lithographic processes involving device fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing light is coated onto a substrate, e.g., a silicon wafer, that is being processed to form a plurality of devices. The coating material, i.e., the resist, is then subjected to light that has been passed through a mask material so that the light reaching the resist produces an image that, after development, yields a desired pattern that is to be transferred into the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with ultraviolet or visible light is somewhat poorer than that achieved with methods such as electron lithography.
Improved resolution with a blanket exposure is achievable by using deep ultraviolet light. One such approach involves a photoresist sensitive to deep ultraviolet radiation containing a compound that produces an acid moiety upon irradiation with such radiation together with a polymer that reacts with the generated acid. Typical acid generator/acid sensitive polymer combinations include an onium salt as the photosensitive acid generator and a polymer such as poly(p-t-butoxycarbonyloxystyrene) as the polymer having a reactive substituent.
The use of an inorganic salt such as the onium salt as the acid generator is not entirely desirable. There is a possibility of contamination of the device being processed by inorganic ionic species from the salt. Additionally, ionic acid generators also have the tendency to phase separate from the acid sensitive resin. Therefore, organic acid generators having reasonable sensitivity to deep ultraviolet light for use in a photoresist are quite desirable.
An organic photoacid generator based on ortho nitrobenzyl esters has been disclosed in U.S. Pat. No. 5,135,838, issued Aug. 4, 1992. These photoacid generators do not present the difficulties associated with inorganic salts and have shown excellent properties for use in applications such as chemically amplified resists. Despite the excellent qualities of these photoacid generators, improvement is always desirable.